Abstract:

Cetacean watching from tour boats has increased in recent years and has been promoted as an ethically
viable alternative to cetacean viewing in captive facilities or directed take. However, short- and longterm
impacts of this industry on the behaviour and energetic expenditure of cetaceans have been
documented. Although multiple studies have investigated the acoustic response of dolphins to marine
tourism, there are several covariates that could also explain some of these results and should be
considered simultaneously. Here, we investigated whether common bottlenose dolphins, Tursiops
truncatus, inhabiting Walvis Bay, Namibia vary their whistle parameters in relation to boat presence,
surface behaviour and/or group composition. We detected an upward shift of up to 1.99 kHz in several
whistle frequency parameters when dolphins were in the presence of one or more tour boats and the
research vessel. No changes were demonstrated in the frequency range, number of inflection points or
duration of whistles. A similar, although less pronounced difference was observed in response to engine
noise generated by the research vessel when idling, suggesting that noise alone plays an important role
in driving this shift in whistle frequency. Additionally, a strong effect of surface behaviour was observed,
with the greatest difference in whistle parameters detected between resting and other behavioural states
that are associated with higher degrees of emotional arousal. Group composition also contributed to the
variation observed, with the impact of boats dependent on whether calves were present or not. Overall
these results demonstrate high natural variation in the frequency parameters of whistles utilized by
dolphins over varying behavioural states and group composition. Anthropogenic impact in the form of
marine tour boats can influence the vocalization parameters of dolphins and such changes could have a
long-term impact if they reduce the communication range of whistles or increase energy expenditure.